The feasibility would be explored of extending the quantitative histological study of the rat gastric mucosa to include investigation of cGMP, along with the cAMP as originallly planned, in order to clarify their separate, and possibly interrelated roles, in the various histological zones. If feasible to determine both cGMP and cAMP simultaneously in the same microtome section tissue sample, the effects on the cGMP and cAMP levels in the histological regions of the gastric mucosa of in vivo treatments to stimulate and inhibit secretion would be followed, instead of the study of cAMP alone. Histidine, the precursor, and pyridoxal PO4, the decarboxylase coenzyme, of the histamine synthesizing system would also be measured in the histological regions under the conditions and following the drug treatments, as above, in the continuing search for limiting factors in the biosynthesis of histamine.